feat: add Newsletter Campaigns backend infrastructure

- Add CampaignManager.php with CPT registration, CRUD, batch sending
- Add CampaignsController.php with 8 REST endpoints (list, create, get, update, delete, send, test, preview)
- Register newsletter_campaign event in EventRegistry for email template
- Initialize CampaignManager in Bootstrap.php
- Register routes in Routes.php

includes/Core/Bootstrap.php

@@ -22,6 +22,7 @@ use WooNooW\Core\DataStores\OrderStore;
 use WooNooW\Core\MediaUpload;
 use WooNooW\Core\Notifications\PushNotificationHandler;
 use WooNooW\Core\Notifications\EmailManager;
+use WooNooW\Core\Campaigns\CampaignManager;
 use WooNooW\Core\ActivityLog\ActivityLogTable;
 use WooNooW\Branding;
 use WooNooW\Frontend\Assets as FrontendAssets;
@@ -40,6 +41,7 @@ class Bootstrap {
         MediaUpload::init();
         PushNotificationHandler::init();
         EmailManager::instance(); // Initialize custom email system
+        CampaignManager::init(); // Initialize campaigns CPT
         
         // Frontend (customer-spa)
         FrontendAssets::init();